Astronomy & Astrophysics manuscript no. Semkov c ESO 2021 July 7, 2021 Letter to the Editor The large amplitude outburst of the young star HBC 722 in NGC 7000/IC 5070, a new FU Orionis candidate E. H. Semkov1, S. P. Peneva1, U. Munari2, A. Milani3, and P. Valisa3 1 Institute of Astronomy and National Astronomical Observatory, Bulgarian Academy of Sciences, 72 Tsarigradsko Shose blvd., BG-1784 Sofia, Bulgaria, e-mail: [email protected] 2 INAF Osservatorio Astronomico di Padova, Sede di Asiago, I-36032 Asiago (VI), Italy 3 ANS Collaboration, Astronomical Observatory, I-36012 Asiago (VI), Italy Received ; accepted ABSTRACT Context. The investigations of the photometric and spectral variability of PMS stars are essential to a better understanding of the early phases of stellar evolution. We are carrying out a photometric monitoring program of some fields of active star formation. One of our targets is the dark cloud region between the bright nebulae NGC 7000 and IC 5070. Aims. We report the discovery of a large amplitude outburst from the young star HBC 722 (LkHα 188 G4) located in the region of NGC 7000/IC 5070. On the basis of photometric and spectroscopic observations, we argue that this outburst is of the FU Orionis type. Methods. We gathered photometric and spectroscopic observations of the object both in the pre-outburst state and during a phase of increase in its brightness. The photometric BVRI data (Johnson-Cousins system) that we present were collected from April 2009 to September 2010. To facilitate transformation from instrumental measurements to the standard system, fifteen comparison stars in the field of HBC 722 were calibrated in the BVRI bands. Optical spectra of HBC 722 were obtained with the 1.3-m telescope of Skinakas Observatory (Crete, Greece) and the 0.6-m telescope of Schiaparelli Observatory in Varese (Italy). Results. The pre-outburst photometric and spectroscopic observations of HBC 722 show both low amplitude photometric variations and an emission-line spectrum typical of T Tau stars. The observed outburst started before May 2010 and reached its maximum brightness in September 2010, with a recorded ∆V∼ 4m. 7 amplitude. Simultaneously with the increase in brightness the color indices changed significantly and the star became appreciably bluer. The light curve of HBC 722 during the period of rise in brightness is similar to the light curves of the classical FUors - FU Ori and V1057 Cyg. The spectral observations during the time of increase in brightness showed significant changes in both the profiles and intensity of the spectral lines. Only Hα remained in emission, while the Hβ, Na I 5890/5896, Mg I triplet 5174, and Ba II 5854/6497 lines were in strong absorption. Key words. stars: pre-main sequence – stars: variables: T Tauri, Herbig Ae/Be – stars: individual: HBC 722 1. Introduction magnitudes amplitude (∆V≈3-5). During these events, the cool spectrum of the quiescence is veiled, and strong emission lines Photometric variability is a fundamental characteristic of pre- from single ionized metals are observed together with reversed main sequence (PMS) stars. One of the most interesting phe- P-Cyg absorption components (Herbig 2007; Aspin et al. 2010). nomena of early stellar evolution is the large increase in bright- ness (usually termed an outburst) that has been recorded for During an optical photometric monitoring of star-forming re- some PMS stars. These outbursts can be grouped into two gions, we discovered in August 2010 a large amplitude outburst main types, named after their respective prototypes: FU Orionis from a T Tauri star located in the dark clouds (so-called ”Gulf of (FUor; Ambartsumian 1971) and EX Lupi (EXor; Herbig 1989). Mexico”) between NGC 7000 (the North America Nebula) and Both typeof stars seem to be related to low-mass PMS objects (T IC 5070 (the Pelican Nebula). The announcement of the out- Tauri stars) with massive circumstellar disks, and their outbursts burst discovery was made by Semkov & Peneva (2010a), and arXiv:1011.1647v1 [astro-ph.SR] 7 Nov 2010 are generally attributed to infall into the central star of material Munari et al. (2010) provided a preliminary report of the spec- from a circumstellar disk (Hartmann & Kenyon 1985). troscopic characteristics of the outburst at optical wavelengths. Only a few FUor objects are known, which all share the Semkov & Peneva (2010b) and Leoni et al. (2010) reported ad- same defining characteristics: a ∆V≈4-5 mag outburst ampli- ditional details of infrared and optical photometry of the out- tude, an F-G supergiant spectrum during outbursts, association burst. The star has coordinates R.A. = 20h58m17s.03 (J2000) and with reflection nebulae, location in star-forming regions, a strong DEC. =+43◦53′43′′. 4 (J2000) and is a member of a small group LiI 6707 Å in absorption, Hα and Na I 5890, 5896 Å displaying of Hα emission objects in the vicinity of LkHα 188, a region P-Cyg profiles, and CO bands in near-infrared spectra (Herbig characterized by active star formation. No other outburst has 1977; Reipurth 1990).The outbursts of FUor objects last for sev- been recorded from this object, for which little is known about eral decades, and the rise time is faster than the decline. the photometric behavior quiescence. The only available spec- EXor objects display frequent, irregular and relatively brief troscopic study of this object was published by Cohen & Kuhi outbursts (lasting from a few months to a few years) of several (1979), who observed the star in 1977 when it was in quies- cence. They classified the absorption spectrum as K7-M0 and Send offprint requests to: E. Semkov ([email protected]) listed the equivalent widths of the Hα,Hβ, and [OI] 6300 spec- 2 E. H. Semkov et al.: The large amplitude outburst of the young star HBC 722 in NGC 7000/IC 5070 tral lines, the only three lines visible in emission in their spec- tra. Cohen & Kuhi (1979) designated the star as LkHα 188 G4. It is included in the catalogs of emission-line stars compiled by Herbig & Bell (1988, with designation HBC 722), and by Kohoutek & Wehmeyer (1999, where it is indicated as [KW97] 53-18). It was also recorded by the 2MASS survey as source 20581702+4353433, which was measured at J=13.205 ±0.012, H=12.136 ±0.052, and K=11.412 ±0.016 on 2000 June 10. 2. Observations We present BVRI photometric data collected from April 2009 to September 2010. Our photometric observations were performed with the 50/70/172 cm Schmidt telescopes of the National Astronomical Observatory Rozhen (Bulgaria) and the 1.3-m Ritchey-Chretien telescope of the Skinakas Observatory1 of the Institute of Astronomy,University of Crete (Greece). We used an ANDOR DZ 436 CCD camera with the 1.3-m RC telescope, and a FLI PL 16803 CCD camera with the Schmidt telescope. All frames were exposed through a set of standard Johnson-Cousins filters. Aperture photometry was performed using DAOPHOT routines. Fig. 1. Finding chart for the BVRI comparison sequence around Using our photometric observations from 2007, 2008, and HBC 722 2009, a comparison sequence of fifteen stars in the field around HBC 722 was calibrated in BVRI (Semkov & Peneva 2010b). Standard stars from Landolt (1992) were used as a reference. Table 1 contains photometric data for the BVRI comparison se- quence. The corresponding mean errors in the mean are also listed. The stars are labeled from A to O in order of their V- band magnitude. In regions of star formation such as NGC 7000, a great percentage of stars can be photometric variables. Therefore, some of our standard stars may be low amplitude variables and we advise observers to use our photometric se- quence with care. The finding chart of the comparison sequence is presented in Fig. 1. The field is 8′ × 8′, north is at the top and east to the left. The chart is retrieved from the STScI Digitized Fig. 2. CCD frames of HBC 722 obtained with the Skinakas Sky Survey Second Generation Red. Observatory 1.3-m RC telescope through a R filter. Left: on 2009 Figure 2 compares two R-band CCD images obtained be- Jul. 31. Right: on 2010 Aug. 26. The appearance of small reflec- fore and during the outburst of HBC 722. The emergence of a tion nebula around the star is observed on the second frame. weak (presumably reflection) nebula around the star is visible in the outburst image. To minimize the light from the surrounding Table 3. Journal of spectroscopic observations nebula and at the same time avoid contamination from adjacent stars, our photometry was obtained through an aperture of 4′′in Date <UT> exp. disp. range ′′ ′′ radius, and the background annulus extended from 13 to 19 . (sec) (Å/pix) (Å) The results of our photometric observations of HBC 722 are summarized in Table 2. The columns provide the JD of observa- 2001-09-10 01:01 3600 1.04 5182-7265 2010-08-21 22:06 3600 2.13 4400-7800 tion, IRVB magnitudes of HBC 722, and the telescope we used. m m 2010-09-02 01:08 14400 2.13 4210-8850 Typical errors in the reported magnitudes are 0. 01-0. 02 for I 2010-09-11 21:12 14400 0.72 5670-7230 and R-band data, 0m. 02-0m. 06 for V, and0m. 03 − 0m. 10 for B-band. A pre-outburst spectrum of HBC 722 was obtained with the 1.3-m RC telescope of Skinakas Observatoryon 2001 September following the recipes of Zwitter and Munari (2000). The spec- 10. The observational procedure and data reduction process tra were calibrated into absolute fluxes by observations of some are described in Semkov (2003).